Liquid crystal displays (LCDs) typically contain a display cell consisting of two sandwiched substrates which each carry at their inner surface a patterned electroconductive layer and a liquid crystal alignment layer. These substrates are kept apart by so-called spacers and the volume thus obtained is filled with a liquid crystal composition. The orientation of the liquid crystal molecules therein is determined by an interaction with the liquid crystal alignment layer which may contain anisotropically oriented polymer molecules. Upon application of an electric field, the orientation of the liquid crystal molecules can be switched from one orientation to another, and a modulation of the light output through crossed polarisers is thereby obtained.
Generally, the substrates in LCDs consist of glass. At present, several key display technologies are being developed in the industry to make flexible displays wherein plastic foils can be used as a substrate. A truly flexible display should not contain inorganic layers on the plastic substrate, since the brittleness of inorganic compositions causes the formation of defects upon bending the display. As a replacement of indium-tin oxide (ITO), the most commonly used electroconductive composition, conducting polymers such as polythiophene can be used. Such replacement of inorganic layers by organic equivalents enables the use of cheaper, easier-to-build, roll-to-roll coating methods for making flat panel displays.
The liquid crystal alignment layers in most of today's LCDs are oriented polyimide (PI) layers. Known since the very beginning of LCD technology, these PI layers have remained essentially unchanged for 25 years. The method of making PI alignment layers is complex and requires careful control of many parameters which may affect the final quality of the display. Typically, the following steps are needed to obtain a PI alignment layer: (1) cleaning the substrate, effected through a sequence of several substeps such as supersonic washing in aqueous solutions, rinsing, supersonic washing in pure water, rinsing, supersonic washing in an organic solvent, blowing with nitrogen, drying, and finally UV photo-cleaning; (2) spin coating the PI precursor (a solution of PI monomers in an organic solvent) and baking to cure the coated layer, typically at a temperature between 200 and 350° C.; and (3) orientation of the PI molecules by stretching or shearing techniques, or more preferably, by rubbing with a rayon, cotton or velvet cloth. The baking step is generally performed in vacuum, otherwise the PI alignment layer does not adhere well to the substrate and may be disrupted during rubbing, especially at the areas of the patterned electroconductive layer at which the ITO layer has been etched out.
The high temperature required during the baking step as well as the use of various organic solvents renders these prior art methods incompatible with many plastic substrates. Other problems are associated with the low stability of the PI precursor (must be stored at low temperature) and the disposal of organic solvents and other chemicals which are necessary in these conventional methods. The build-up of electrostatic charges in the PI layer, e.g. during rubbing, is a particularly serious problem as dust particles are attracted thereby, which, once trapped in the display cell, may cause poor alignment, severe wedging of the substrates or electrical breakdown by short circuiting across the dust particle.
In order to solve these problems, alternative methods have been described to obtain LCD alignment layers. Photo-alignment methods such as the anisotropic cross-linking of poly(vinyl cinnamate) and PI films by exposure to linearly polarised UV light have been described (Applied Physics Letters, volume 73, p. 3372, published in 1998). Such methods are also suitable for aligning polythiophene layers, but are not a suitable alternative for conventional PI layers because of their thermal instability. The problem of electrostatic charge generation may also be solved by making electroconductive alignment layers as described in U.S. Pat. No. 5,639,398, which discloses the coating of a viscous lyotropic polyaniline solution on ITO and then orienting by shearing with a blade of a knife or a glass plate. While drying, the liquid crystalline polyaniline molecules retain their orientation. However, the conductivity values of the polyaniline layer reported in U.S. Pat. No. 5,639,398 are low, so ITO is still needed as an electrode layer.
U.S. Pat. No. 5,465,169 discloses a liquid crystal device, comprising a pair of substrates each having an electrode thereon and a liquid crystal disposed between the substrates, wherein at least on of the substrates is provided with an electroconductive protective film and also an alignment film comprising an alignment material and a polymeric electroconductive compound. This polymeric electroconductive compound may preferably be a basic polymer, examples of which may suitably include polypyrrole, polyaniline and derivatives thereof represented by formula (1) and (2), and polythiophene and derivatives thereof. According to the invention of U.S. Pat. No. 5,465,169 it is preferred that the alignment material constituting the alignment film comprises a compound having an acidic functional group so as to form a polymer complex having an improved electroconductivity between the alignment material and the polymeric electroconductive compound, suitable examples of such alignment material including polyimides, polyamideimides and precursors thereof.
EP-A 449 047 discloses a liquid crystal device comprising a pair of opposing substrates and a liquid crystal rendering a chiral smectic phase, disposed between the pair of substrates, wherein at least one of the substrates is provided with an alignment film comprising a polymer containing a skeleton selected from the group consisting of acetylene, phenylene, phenylenevinylene, phenylenexylidene, benzyl, phenylene sulfide, dimethylparaphenylene sulfide, thienylene, furan, selenophene, vinylpyridine, vinylnaphthalene, vinylferrocene, vinylcarbazole, phenylene oxide, phenylene selenide, heptadiyne, benzothiophene, thiophen, pyrrole, aniline and naphthylene. However, experimental evidence is only provided for polymer produced with p-xylylene and various polyparaphenylene precursors.
D-E. Seo, S. Kobayashi, M. Nishikawa and Y. Yabe in Journal of the Japanese Journal of Applied Physics, volume 35, pages 3531-3532, published in 1996, disclosed the dependence of the obtaining of pre-tilt angles in nematic liquid crystals on rubbed poly(3-alkyl-thiophene) surfaces upon alkyl chain length and established that the pre-tilt angle was less than 2° for alkyl chains with 1 to 8 carbon atoms, up to 5° depending on the rubbing strength for alkyl chains with 9 carbon atoms, up to 38° depending on the rubbing strength for alkyl groups with 10 carbon atoms and up to 70-80° depending on the rubbing strength with alkyl groups with 11 or 12 carbon atoms.
Currently used liquid crystal alignment layers require the use of high temperatures and/or the use of organic solvents or other hazardous chemicals and in general have to be used in association with an electroconductive compound. This rules out the use of many polymeric substrate materials in liquid crystal devices.